Embrittlement-resistant iron-chromium-aluminum-yttrium alloys



Jan. 17, 1967 c. s.wuKUslcK 3,298,826

EMBRITTLEMENT-RES ISTANT IRON- GHROMIUM-ALUMINUM-YTTRIUM ALLOYS Filed April 6, 1964 lCarl S. Wukusick ATTORNEY.

United States Patent O Atomic Energy Commission Filed Apr. 6, 1964, Ser. No. 357,845 4 Claims. (Cl. 75--124) This invention relates to oxidation-resistant all-oys and more particularly to alloys of iron, chromium, aluminum and yttrium.

Alloys of iron, chromium, aluminum and yttrium were originally developed for their oxidation resistance in air at temperatures over 2000 F. As described in U.S. Patent 3,027,252, issued March 27, 1962, to James A. McGurty and lohn F. Collins, the useful range of compositions for these alloys was as follows: 20.0 to 95.0 weight percent chromium, 0.5 to 4.0 weight percent aluminum, 0.5 to 3.0 weight percent yttrium and the balance iron.

These alloys have also been found highly resistant to oxidation :and corrosion by superheated steam and thus potentially useful in solving a critical problem in nuclear reactor technol-ogy. One of the most 4promising approaches for obtaining increased efficiency in the generation of power by nuclear reactors is the use of a superheated steam system wherein superheating is effected by direct passage of steam through the reactor core. Development of this type reactor has been hampered by the lack of suitable fuel-element cladding and structural materials. Stainless steels and other metals which have been employed for conventional superheated-steam equipment such as turbines are unsatisfactory for prolonged use in a nuclear reactor superheated steam environment. Oxidation and corrosion of these metals are accelerated by the presence of free oxygen produced by radiolytic decomposition of water. In addition, neutron economy imposes a severe limitation on cladding thickness so that better resistance is required than for conventional applications.

While satisfactory with regar-d t-o oxidation and corrosion by superheated steam, the above-described ironchromium-aluminum-yttrium alloys are defective in that their mechanical properties are adversely affected by holding .at the temperatures encountered in superheated-steam react-or service. These alloys become severely hardened and embrittled within several hours at` temperatures of about 650 to 1000 F., and prolonged holding at temperatures from 1000 to 1300 F. may also result in embrittlement. Typical superheated steam nuclear reactor systems require :an operating steam temperature of about 900 to 1050 F. an-d a fuel element temperature up to about 125 0 F. The high probability of embrittlement and mechanical failure thus precludes use of existing ironchromium-aluminum-yttrium alloys in such systems. It is desired to eliminate or minimize the embrittlement characteristic of these alloys while retaining their resistance to oxidation and corrosion.

It is therefore an object of my invention t provide an alloy suitable for use in contact with superheated-steam in nuclear reactors.

Another object is to provide an iron-chromium-aluminum-yttrium alloy which is resistant to embrittlement at temperatures 0f 650 to 1300 F.

Another object is to provide an embrittlement-resistant ironchromium-aluminum-yttrium alloy which retains a high degree of resistance to -oxidation and corrosion.

Other objects and advantages of my invention will be apparent from the following detailed description and claims.

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In accordance with my invention a novel alloy having the following composition is provided: 0 to 20 weight percent chromium, 0.5 to 12 weight percent aluminum, 0.1 to 3.0 weight percent yttrium and the balance iron. This alloy is less susceptible to embrittlement than previously known iron-chromium-aluminum-yttrium alloys, but it exhibits substantially the same superior resistance to oxidation and corrosion.

I have found that embrittlement of iron-chromiumaluminum-yttrium .alloys is avoided by reducing the chromium content of a level below the previous minimum. Although my invention is not to be understood as limited to 4a particular theory, it is postulated that embrittlement below 550 C. is caused by precipitation of a chromiumrich ferrite phase and above 550 C. by precipitation of an iron-chromium sigma phase and that precipitation of these phases is significantly lessened in the temperature range of interest at chromium levels below 20 weight percent. l have also found that yttrium renders ironchromium-.aluminum alloys resistant to oxidation over a wider range of compositions than was previously believed effective. Oxidation resistance is maintained at low levels of chromium, and even in chromium-free alloys.

Chromium is provided in the alloy .at a level of 0 to 20 weight percent. At higher levels the alloy is subject to embrittlement as described above. An intermediate level of 5 to 15 weight percent chromium is preferred in order to retain to the maximum extent the favorable properties of the previous iron-chromium-aluminum-yttrium alloys, while avoiding embrittlement.

An aluminum content of at least 0.5 weight percent is required for oxidation resistance, and above 12 weight percent aluminum the alloy becomes embrittled. At the preferred chromium content an aluminum content of 4 to 6 weight percent is preferred for maximum workability and oxidation resistance. For chromium-free alloys an aluminum content of 5 to 7.5 weight percent is preferred.

The yttrium content of the alloy may be varied from 0.1 to 3.0 weight percent. At lower levels the alloy loses its oxidation resistance and at higher levels an excessive amount of second phase limits fabricability. About 1 weight percent is preferred.

The alloy composition disclosed and claimed herein may be fabricated into usable shapes by means of conventional -techniques such as extrusion, rolling and drawing.

My invention is further illustrated by the following examples.

Example I Alloy specimens of the weight composition iron-25 percent chromium-4 percent aluminum-1 percent yttrium and iron-15 percent chromium-4 percent aluminum-1 percent yttrium were prepared in 6.3 millimeter diameter rod form by swaging of extruded rods. The specimens were heated in air at a temperature of 450 C. for a period of over 200 hours. Hardness of the specimens was measured periodically throughout the heating period. The results obtained may be seen by reference to the accompanying figure wherein hardness is plotted as a function of time at temperature. The hardness of the 25 percent chromium specimen showed a significant increase while the 15 ipercent chromium specimen was increased only slightly.

Example Il Alloy specimens of the composition given in Example I were tested by'exposure to superheated steam :at 730 C. and 1150 C. The results obtained may be seen by reference to the following table.

It may be seen from the above that the superheatedsteam corrosion resistance of the percent chromium alloy is similar to the resistance of the percent chromium alloy.

Example III Alloy specimens of the composition iron-15 weight percent chromium-4 weight percent aluminum-1 Weight percent yttrium .and the balance iron were prepared by induction melting, extruding at 1000" C. into rods, hotpress-forging the rods into sheet, hot-rolling, and cold rolling the sheet to a thickness of 0.76 millimeter. Specimens were tested for oxidation resistance by heating in air at temperatures of 450, 650, 750, 900, 1100, and 1300o C. for 1000 hours. One set of specimens was tested in the cold-rolled condition and another set was annealed at 1000 C. prior to testing. All of the specimens maintained their integrity in these tests, with an adherent oxide coating being formed on the surface.

Example IV TABLE II.-OXIDATION TESTING OF ALLOY SPECIMENS Centuneter Alloy Composition (weight percent) Test Temperature, Test Temperature,

C, 200 hours C, 500 hours Fe-l() Cr-4 Al 09 0 .11 .15 (l) (2) 11e-10 Cr-4 Al-l Y 18 0 03 13 96 (3) Fe-5 Cr-6 .Al-1 Y 1l 02 09 15 93 4. 5 Fe-7.5 Al-l Y. 07 .21 05 .30 98 4. 9 1re-25 (Jr-4 Ar-i Y .01 01 .01 .1G 1. 5 3. 7 50 It may be seen from the above that a range of alloy compositions exhibit excellent oxidation resistance when stabilized by the presence of yttrium. The specimen without yttrium failed at 1100 C. and above. Substantially the same results were -obtained for the chromium-free alloy as for the chromium-containing alloys, thus demonstrating that chromium is not critical to oxidation resistance.

The above examples are merely illustrative and are not to be understood as limiting the scope of my invention, which is limited only as indicated by the appended claims.

Having thus described my invention, I claim:

1. An alloy of iron, chromium, .aluminum and yttrium which consists of 0 to 15 Weight percent chromium, 0.5 to 12 weight percent aluminum, 0.1 to 3 percent yttrium and the balance iron.

2. An alloy of iron, chromium, aluminum and yttrium which Iconsists of 5 to 15 weight percent chromium, 0.5 to 12 Weight percent aluminum, 0.1 to 3 percent yttrium and the balance iron.

3. An alloy resistant to embrittlement in service at a temperature of 650 F. to 1300 F., said .alloy consisting of 5 to 15 Weight percent chromium, 4 to 6 weight percent aluminum, 0.1 to 3 Weight percent yttrium and the balance iron.

4. An alloy consisting of 0.5 to 12 weight `percent aluminum, 0.1 to 3.0 Weight percent yttrium and the balance iron.

References Cited by the Examiner UNITED STATES PATENTS 2,043,631 6/1936 Scheil 75--124 2,061,370 11/1936 Rohn 75-124 2,105,283 l/1938 Godecke --124 2,190,486 2/ 1940 Schafmeister 75-128 2,191,790 2/1940 Franks 75-124 X 3,027,252 3/1962 McGurty 75-126 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examinez'. 

1. AN ALLOY OF IRON, CHROMIUM, ALUMINUM AND YTTRIUM WHICH CONSISTS OF 0 TO 15 WEIGHT PRCENT CHROMIUM, 0.5 TO 12 WEIGHT PERCENT ALUMINUM, 0.1 TO 3 PERCENT YTTRIUM AND THE BALANCE IRON. 